This happened to me a few months ago, somehow, it was caused by my front O2 sensor being unplugged magically. Anyways, I replaced the plugs/wires, fuel filter, front O2 sensor, and eveything was peachy. MAF was also clean as a whistle when I looked.

Now what happens is the car will idle VERY rough. The idles will start off high (1500 range) and fluctuate, up/down, up/down, and finally settle around 500, where the car seems like it might stall, but it never does...never has stalled either.

When I'm driving along, it feels like the gas is giving in and out a few times a second, on off on off...even when I keep the pedal steady. To make the problem go away, I floor it, for a second it hiccups then the car takes off like normal.

People are recommending replacing the IAC...but don't IAC problems usually occur when the car needs to be given gas while started?

When I let off the gas, the car backfires like CRAZY...it's embarrassing.

Any help is appreciated.

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You've most likely got an exhaust leak, or possibly a hose loose between the TB and air filter, or a vaccuum line came of. Check the vaccuum lines and intake first because that's easy, but my guess is that you've got an exhaust leak where the h-pipe bolts to the passenger side header. Those little gaskets are notorious for going out.

I don't think it could be an exhaust leak, only because I had my UPR O/R X done professionally. The problem was here once before and went away with a new O2 sensor, but now it came back...wouldn't the exhaust leak cause the problem to always be here?

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Scope the engine. All failures will be exposed that way without beating around the bush all day. Bad O2 sensors will show up in an instant. I had to replace all my O2 sensors in my '98 GT before it would run right. Some kind of contamination screwed up the front O2's and running over road rash ripped my back ones right off.

Also check under the hood for loose anything. Stray wires, sensors connectors, vacuum hoses, etc. A lot can get messed up in a hurry with today's electornically-controlled engines. Even your gas cap not screwed on tight can make a check engine light come on.

By the way, with all that is going on with your car, doesn't any engine light come on? A dealer is, yes, more expensive but will be your best bet to check the engine thoroughly as he has all the official Ford bulletins explaining known engine problems and their fixes.

You said you replaced your forward O2 sensors, what about the back. My '98 has four total. Two before the cats and two after the cats. The back ones can get damaged easily by running over stuff like what happened to me. Also check the wiring to both rear sensors to be sure something didn't get shaken loose.

If you can't find anything, take it to the dealer. You may even learn more quirks about your engine than you suspect. It is worth it at least once to have your system thoroughly scoped and everything properly set-up. Good luck.

I'm going to take a look under the car tomorrow and check the O2s. I heard that the front O2s are the ones that affect how the car runs, and the rears are used just to make sure the cats are there and working. Heard that the rear O2s just trigger the SES light, and thats it.

I don't have a SES/CEL light on, I don't know why. I probably should the way the car is running. It's ridiculous. Would my car still be throwing codes even without the light lit?

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Yes it will have codes.
Surging and backfiring could be a ton of stuff- this is a short list leaking intake, bad valve, out of time, streched timming chain, bad coils, loose spark plug wire, bad spark plug, bad injector.......ect. Get the codes checked.
You can take it to AutoZone and they will check the codes for free.
Later,
Eman

Yes it will have codes.
Surging and backfiring could be a ton of stuff- this is a short list leaking intake, bad valve, out of time, streched timming chain, bad coils, loose spark plug wire, bad spark plug, bad injector.......ect. Get the codes checked.
You can take it to AutoZone and they will check the codes for free.
Later,
Eman

Should I get them checked even though the car isn't throwing a light?

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Yes check the codes anyway. It's free and it will help. it'll give you a good starting place. It is possible that an exhaust leak could cause your symptoms. It will cause the O2 sensors to sample gas that is not a good representation of exhaust. Then the comp. will try to correct for conditions that don't exist. but that will useally throw a mil code. I would bet on an intake leak from your symptoms. good luck and let us know what happens.

Yes check the codes anyway. It's free and it will help. it'll give you a good starting place. It is possible that an exhaust leak could cause your symptoms. It will cause the O2 sensors to sample gas that is not a good representation of exhaust. Then the comp. will try to correct for conditions that don't exist. but that will useally throw a mil code. I would bet on an intake leak from your symptoms. good luck and let us know what happens.

Well that was a blast.

SES light isn't on, but car threw 10 codes.

Threw the rear O2 sensor malfunction code 3 different times, and he said the rest were all rear O2 sensor related. I have an O/R pipe without mils, so that explains the rears. MILs wouldn't fix how my car is running though, so looks like the exhaust is out of the picture.

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sounds like an intake leak then. check all hoses and connections. if they're all tight then start spraying starting fluid on all air path joints while the car is running. when it stalls, there's your leak.

sounds like an intake leak then. check all hoses and connections. if they're all tight then start spraying starting fluid on all air path joints while the car is running. when it stalls, there's your leak.

Car has never stalled though, so hm.

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Are the O2 sensor wires correctly plugged in (R to R and L to L)? If they are swapped, the car will run poorly.

The backfiring can be due to an overly rich condition. So, unplug the MAF and see how it runs (idles); it will be working from tables of info stored in the PCM (for a failed MAF). If it runs better, then the MAF is bad. If it runs the same (as now), then the MAF can be ruled out as a culprit.

Lastly, don't rule out the IAC. Take it off (watch that you don't loose or rip the gasket) and give it a good cleaning with carb cleaner and a small brush. When the carb cleaner has dried off, tap the IAC on the workbench to loosen the scale. Repeat the spray/dry/tap routine until most of the dirt has been dislodged. If this helps, you know it is the IAC.

Are the O2 sensor wires correctly plugged in (R to R and L to L)? If they are swapped, the car will run poorly.

The backfiring can be due to an overly rich condition. So, unplug the MAF and see how it runs (idles); it will be working from tables of info stored in the PCM (for a failed MAF). If it runs better, then the MAF is bad. If it runs the same (as now), then the MAF can be ruled out as a culprit.

Lastly, don't rule out the IAC. Take it off (watch that you don't loose or rip the gasket) and give it a good cleaning with carb cleaner and a small brush. When the carb cleaner has dried off, tap the IAC on the workbench to loosen the scale. Repeat the spray/dry/tap routine until most of the dirt has been dislodged. If this helps, you know it is the IAC.

Good luck.

Sensors in the front are 100% plugged in correctly. Sensors in the rear...well yeah I only have one, and the other wire is taped up...but I've heard from multiple people that the rears don't affect performance, that they just trip the SES light for emissions.

I just talked to a guy at work who is VERY familiar with mustangs (5.0s mostly), and he told me that since the car is bucking while driving steadily that the engine is making a rich/lean/rich/lean mixture. He also is betting that it's the O2 or IAC. I'm going to clean the IAC after work, and I might as well go try the MAF sensor now.

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Sorry, I was unclear. when you spray starting fluid on a leaking, running engine it will get sucked in the leak and stall out the engine. It is a method of finding the leak.

also I know you probably checked but when you did the exhaust did you swap the front o2's left for right? For some dumb reason ford has different sensors in the two sides, so if you put the right hand sensor on the left and vice-versa they won't work.

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